Switch to Github actions for CI

Author: marijnh

.github/workflows/ci.yml

@@ -0,0 +1,13 @@
+name: ci
+on: [push]
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+    name: Build and test
+    steps:
+      - uses: actions/checkout@v1
+      - uses: bahmutov/npm-install@v1
+        with:
+          useLockFile: false
+      - run: npm test
+      - run: npm run test:test262

.travis.yml

@@ -1,6 +1,6 @@
 # Acorn
 
-[![Build Status](https://travis-ci.org/acornjs/acorn.svg?branch=master)](https://travis-ci.org/acornjs/acorn)
+[![Build Status](https://github.com/acornjs/acorn/workflows/ci/badge.svg)](https://github.com/acornjs/acorn/actions)
 [![NPM version](https://img.shields.io/npm/v/acorn.svg)](https://www.npmjs.com/package/acorn)
 [![CDNJS](https://img.shields.io/cdnjs/v/acorn.svg)](https://cdnjs.com/libraries/acorn)